Strings of light emitting diodes (LEDs), such as those used in advanced automotive headlights, also referred to as “matrix” headlights, use an LED driver to control the brightness of the headlight. Pulse Width Modulation (PWM) dimming is often used to control the brightness of the LED string. Each LED in the LED string is configured to be controlled individually in order to control the overall brightness of the headlight. To implement PWM dimming in an LED driver, a boost stage is generally used to boost the voltage, resulting in a two-stage system.
A typical LED driver 100 that uses PWM dimming for each individual LED is shown in FIG. 1. The LEDs (D112, D114, . . . D118, D120) are connected in series as an LED string 110. Although four LEDs are illustrated in FIG. 1, any number of LEDs can be provided in the LED string 110. There is a bypass switch (S112, S114, . . . S118, S120) in parallel with each LED (respectively, D112, D114, . . . D118 ,and D120) under control of a controller 130. The LED driver is powered by a DC voltage source 140 in conjunction with a DC current source 150. If a switch is open (e.g., S112), the voltage across the corresponding LED (e.g., D112) is greater than a threshold voltage, and thus the current flows through the LED and it emits light. On the other hand, when the switch is closed, the voltage across the corresponding LED is less than the threshold so the LED is turned off. By selecting the relative duration of on and off times, the average brightness of each LED can be individually controlled, which sometimes is referred to as PWM dimming